chore: minor prototype fixes (#15907)

* chore: minor prototype fixes

* Fixes an e2e test failure in the slide toggle prototype.
* Fixes the underlines not being reset in the menu item.

* add comment for CSS override

Author: crisbeto

src/material-experimental/mdc-menu/menu.scss

@@ -37,6 +37,7 @@
   color: inherit;
   font-size: inherit;
   background: none;
+  text-decoration: none;
 
   &[disabled] {
     cursor: default;

src/material-experimental/mdc-slide-toggle/slide-toggle.scss

@@ -40,4 +40,14 @@
   .mat-ripple-element {
     opacity: map-get($mdc-ripple-dark-ink-opacities, press);
   }
+
+  // Angular Material supports disabling all animations when NoopAnimationsModule is imported.
+  // TODO(mmalerba): Look into using MDC's Sass queries to separate the animation styles and
+  //  conditionally add them. Consider the size cost when deciding whether to switch.
+  &._mat-animation-noopable {
+    .mdc-switch__thumb-underlay,
+    .mdc-switch__thumb-underlay::before {
+      transition: none;
+    }
+  }
 }